The invention relates generally to non-contact position sensors and more particularly to such sensors which are digital pulse transformer position sensors.
There are many instances in which it is desired to determine the position of a component. Generally, systems for providing this information rely on the interconnection of electromechanical circuits through electrical contacts, i.e., a contact position sensor, or the change in the strength of an electrical characteristic, such as magnetic flux in a transformer system, such as magnetic flux in an LVDT (linear variable displacement transducer) or a Hall effect transducer. The former systems are typically inexpensive but are subject to mechanical wear over time. The latter systems can be expensive, as in the case of the windings of LVDT""s, and are somewhat temperature sensitive requiring some type of temperature compensation or, in the case of systems relying on the use of permanent magnets, are sensitive to the accumulation of ferromagnetic debris which can affect the accuracy thereof, as well as having limitations in the location of the magnet.
An object of the present invention is the provisions of a robust, highly accurate and reliable non-contact position sensor. Another object of the invention is the provision of a position sensor which is free of the above noted limitations of the prior art.
Briefly stated, a position sensor made in accordance with the invention comprises at least one primary or drive coil disposed on a planar surface and at least one differential secondary or sense coil disposed in magnetic field receiving relationship with the drive coil on a planar surface, that is, a sense coil having a pair of loops arranged generally in a figure eight pattern with the cross-over from one loop to the other effected by suitable means to provide oppositely directed induced voltages within the two loops, such as by jumpers, or by vias arranged in a circuit board with interrupted loops on one layer of a board and terminations of the interrupted loops connected to vias having a conductive layer going to another board layer where traces are formed to provide cross over connections. The loops of the sense coil are configured so that the flux generated in one loop substantially equals the flux generated in the other loop and the oppositely directed induced voltages in the two loops cancel, resulting in zero output voltage. In accordance with the invention, a generally planar target of electrically conductive material is movably mounted to pass over the sense coil. At a sufficiently high frequency, the magnetic field will generate eddy currents in the target which distort the magnetic field. The target is moved over one or the other of the loops so that an output of a given polarity is produced. When the target moves through a transition point, the output switches to the opposite polarity.
According to a feature of the invention, power requirements are reduced by providing a pulse excitation in the primary coil, a di/dt of one or two amps/microsecond, on a continuing basis having a selected period, e.g. every millisecond. A measurement taking approximately 1 microsecond is obtained by a detection circuit and is latched for the remainder of the period. The measurement process is then repeated on a continuing basis.
In accordance with the invention, the detection circuit determines whether the signal is greater or less than zero and thus provides a precise indication of the location of the target at the transition point where the output signal flips from one polarity to the opposite polarity.
According to a feature of the invention, individual bits are provided with the relationship between patterns in each bit row and the target being varied to provide transition points at different locations along an axial length enabling binary counting. According to one preferred embodiment, a four bit sensor is provided resulting in 16 measurable positions.
In one preferred embodiment, a separate target portion is moved across each loop. In another preferred embodiment, a narrow target is arranged to pass serially over each loop of the sense coils.
According to a feature of the invention, the loops of the secondary coils are formed symmetrically adjacent to the transition point and can be widened adjacent to the transition point to exaggerate the signal difference and increase sensitivity. According to yet another feature, extra pieces may be provided for one or both loops of a secondary to permit trimming in order to ensure that the total flux produced in one loop will be the same as the other loop of a sense coil. According to a feature of the invention, the pattern of the sense coils can be varied to change the physical location of the transition relative to the overall length of the sense coils, e.g., one loop of the sense coil, while enclosing the same area as the other sense coil loop, can have a long, narrow configuration relative to the other loop. Both loops would preferably still have a symmetrical widening at the transition zone. According to yet another feature, the electronics are preferably provided with hysteresis at least equal to any expected electrical noise to stabilize the output at the transition zone. According to still another feature, a nulling target could be provided over one loop and an enhancing target over the other loop.
According to a preferred embodiment of the invention, a four layer circuit board is used with one outer layer having both primary or drive and secondary or sense coils disposed thereon utilizing photolithographic techniques providing a high degree of accuracy for the coil patterns. A second layer includes the interconnecting traces for the transition zones which cooperate with vias having an electrically conductive lining going through the layers. The vias may be filled with ferromagnetic material to enhance the inductive field. The remainder of the second layer is provided with a ground layer or copper or the like. A third outer layer has the associated electronics disposed thereon including an ASIC with the drive circuit and the detection circuit while the fourth layer is provided with interconnections for the electronics and the remainder having a copper ground layer.
Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description. The objects and advantages of the invention may be realized and attained by the means of the instrumentalities, combinations and methods particularly pointed out in the appended claims.